1. Field of the Invention
The present invention relates to an air fuel mixer for the combustor of a gas turbine engine, and, more particularly, to an air fuel mixer for the combustor of a gas turbine engine which uniformly mixes fuel and air so as to reduce NOx formed by the combustion of the fuel/air mixture.
2. Description of Related Art
Air pollution concerns worldwide have led to stricter emissions standards requiring significant reductions in gas turbine pollutant emissions, especially for industrial and power generation applications. Nitrous Oxide (NOx), which is a precursor to atmospheric pollution, is generally formed in the high temperature regions of the gas turbine combustor by direct oxidation of atmospheric nitrogen with oxygen. Reductions in gas turbine emissions of NOx have been obtained by the reduction of flame temperatures in the combustor, such as through the injection of high purity water or steam in the combustor. Additionally, exhaust gas emissions have been reduced through measures such as selective catalytic reduction. While both the wet techniques (water/steam injection) and selective catalytic reduction have proven themselves in the field, both of these techniques require extensive use of ancillary equipment. Obviously, this drives the cost of energy production higher. Other techniques for the reduction of gas turbine emissions include "rich burn, quick quench, lean burn" and "lean premix" combustion, where the fuel is burned at a lower temperature.
In a typical aero-derivative industrial gas turbine engine, fuel is burned in an annular combustor. The fuel is metered and injected into the combustor by means of multiple nozzles into a venturi along with combustion air having a designated amount of swirl. No particular care has been exercised in the prior art, however, in the design of the nozzle, the venturi or the dome end of the combustor to mix the fuel and air uniformly to reduce the flame temperatures. Accordingly, non-uniformity of the air/fuel mixture causes the flame to be locally hotter, leading to significantly enhanced production of NOx.
In the typical aircraft gas turbine engine, flame stability and variable cycle operation of the engine dominate combustor design requirements. This has, in general, resulted in combustor designs with the combustion at the dome end of the combustor proceeding at the highest possible temperatures at stoichiometric conditions. This, in turn, leads to large quantities of NOx being formed in such gas turbine combustors since it has been of secondary importance.
While premixing ducts in the prior art have been utilized in lean burning designs, they have been found to be unsatisfactory due to flashback and auto-ignition considerations for modern gas turbine applications. Flashback involves the flame of the combustor being drawn back into the mixing section, which is most often caused by a backflow from the combustor due to compressor instability and transient flows. Auto-ignition of the fuel/air mixture can occur within the premixing duct if the velocity of the air flow is not fast enough, i.e., if a significant portion of the residence time distribution of the premixer is above a critical value based on chemical kinetics. Flashback and auto-ignition have become serious considerations in the design of mixers for aero-derivative engines due to increased pressure ratios and operating temperatures.
Other air fuel mixers for gas turbine combustors to provide uniform mixing are disclosed in U.S. Pat. Nos. 5,165,241 and 5,251,447, which are owned by the assignee of the current invention. These air fuel mixers include a mixing duct, a set of inner and outer annular counter-rotating swirlers at the upstream end of the mixing duct, a centerbody, and separate manners of injecting fuel into the mixing duct wherein high pressure air from a compressor is injected into the mixing duct through the swirlers to form an intense shear region for mixing fuel injected into the mixing duct. By contrast, the present invention is principally intended for use in an aeronautical gas turbine engine and provides an air fuel mixer which maximizes mixing without a set of counter-rotating swirlers or a centerbody. However, the air fuel mixer of the present invention could be employed in industrial and other gas turbine engines by suitably modifying design parameters without departing from the scope of the invention.
Accordingly, a primary objective of the present invention is to provide an air fuel mixer for an aeronautical gas turbine engine which avoids the problems of auto-ignition and flashback.
Another objective of the present invention is to provide an air fuel mixer which more uniformly mixes fuel and air without incurring backflow from the combustor.
Yet another objective of the present invention is to provide an air fuel mixer which supplies a significant swirl to the fuel/air mixture so as to generate an adverse pressure gradient in the combustion zone, thereby causing a central recirculation zone which stabilizes the flames.
These objectives and other features of the present invention will become more readily apparent upon reference to the following description when taken in conjunction with the following drawing.